Maint: make VPN connection not specific to OpenVPN

- Add VPN field to ServerSelection struct
- Set VPN type to server selection at start using VPN_TYPE
- Change OpenVPNConnection to Connection with Type field
- Rename Provider GetOpenVPNConnection to GetConnection
- Rename GetTargetIPOpenVPNConnection to GetTargetIPConnection
- Rename PickRandomOpenVPNConnection to PickRandomConnection
- Add 'OpenVPN' prefix to OpenVPN specific methods on connection

internal/provider/utils/pick_test.go

@@ -8,19 +8,19 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-func Test_PickRandomOpenVPNConnection(t *testing.T) {
+func Test_PickRandomConnection(t *testing.T) {
 	t.Parallel()
-	connections := []models.OpenVPNConnection{
+	connections := []models.Connection{
 		{Port: 1}, {Port: 2}, {Port: 3}, {Port: 4},
 	}
 	source := rand.NewSource(0)
 
-	connection := PickRandomOpenVPNConnection(connections, source)
-	assert.Equal(t, models.OpenVPNConnection{Port: 3}, connection)
+	connection := PickRandomConnection(connections, source)
+	assert.Equal(t, models.Connection{Port: 3}, connection)
 
-	connection = PickRandomOpenVPNConnection(connections, source)
-	assert.Equal(t, models.OpenVPNConnection{Port: 3}, connection)
+	connection = PickRandomConnection(connections, source)
+	assert.Equal(t, models.Connection{Port: 3}, connection)
 
-	connection = PickRandomOpenVPNConnection(connections, source)
-	assert.Equal(t, models.OpenVPNConnection{Port: 2}, connection)
+	connection = PickRandomConnection(connections, source)
+	assert.Equal(t, models.Connection{Port: 2}, connection)
 }